Pioneer SX-636: first repair/recap help

[darkhosis]

I hope this is within the forum rules, I decided to start a new thread regarding my SX-636 project. I have been trying to work on something without knowing exactly what I'm looking for, which I don't feel comfortable with. So before I completely ruin this receiver I wanted to back up and ask the forum members for assistance. Also, the original thread - regarding the fuse replacement - was solved so in a sense that issue is resolved. Thanks for your patience and help. shoutout to petehall347 Bassblaster larryderouin and others for the previous advice.

I got this receiver knowing it had an issue with the right channel because I wanted to learn to fix it, and I plan on updating the parts that are recommended. Since this is my first attempt at diagnosing and recapping, I'm wanted to restate the issue so people know I'm coming at this as an eager novice. ( My goal is to be able to confidently work on my Marantz 2230 down the line which I paid considerably more than $10 for)

I think I can solder decent, it's a pencil-type (no temp control), but I have a good tip. I printed out the schematic on big 11x17 paper, but I'm not an expert in reading it. I got DMM, Dim Bulb Tester, flux.

If there's someone willing to help me take my first step (despite some work done already), I'd really appreciate it

 

[darkhosis]

Looks like I'll have some downtime at my job today, so I brought the SX-636 along for an inspection. Maybe there's something I missed, and I can figure out a little better how this thing works, or doesn't work.

Hooked up to the Dim Bulb Tester, this time with a 60w bulb. The DMT doesn't register much at all. Im gathering that this is a good sign.

The problem to solve before recap is with the right channel. Made the mistake of hooking up a speaker last week, horrible sound. Thru the headphones: nothing on the right side but I still hear faint signal on the left side even when the balance is all the way right.
Something blew the right fuse at some point, so now I have new fuses in place.
I've been using the Offest Bias test to measure the DC output. The left is 9mA, and the right is currently measuring around -14 V with some parts removed from the power assembly (AWH-034A).

Not sure what to look for. The AC outlets in back got damaged at some point, pushed in. Took pics of most of the inside for reference and finally the parts I've removed from the Power Assy

 

[darkhosis]

Next day of the video game competition, some more downtime during matches. Working on a receiver that was made at least a couple decades before most of these attendees was born..kind of funny

I took a bunch of measurements with my multimeter yesterday trying to id electrical flow on the schematic. Im still not clear on DC voltage vs DC current vs signal voltage on the schematic. I feel like that's a big step to make understanding where to look.

Second, I've been trying to figure out DC output at the speaker terminals, and it's bugging me. Even though I've been documenting my work it's not adding up. I'm diode testing the transitors and they seem ok but the measurements keep changing
action / right channel
initial / 18.8 V
T03 outputs removed / -21.75 V
Q5 removed / +26 V
Q8 removed / -12.89 V
Q10, C2 removed / -12.28
removed, then resoldered post 5 / 3.38 V

The last one stumps me completely. I was starting to remove solder on Q2 but stopped, measured and ..big change.

 

[BobbyN]

I would first determine what is causing that fuse to blow. I would then determine if the problem is in the power amp stage or before.
Bob

 

[darkhosis]

I have a dumb question. When Im measuring DC coming off the circuit or into the circuit do I always need to have the black on a ground and the red on what I'm trying to measure?

 

[Oldsansui441]

In answer to your question, yes.
Just keep doing as you have been, pull the outputs and diode test first, then if a prob exists check the drivers, pre-drivers and input pair and test with diode test on all, there must have been a reason the fuse blew initially.
Since you will have these small signal transistors out anyway l would replace them with part numbers provided on different threads here on AK for the 636 as some of them are known to fault. Then look at resistors and diodes on that board.
Check the 4 diodes in the main power rectifier also.

 

[zebulon1]

So:
Check every diode, resistor and transistor on the right amp. The ceramics should be good. Replace the electrolytic caps.
Check the outputs using EW's thread.

Bipolar Junction Transistor Testing Basics

Don't run the set until you get the amp sorted and populated with good or new components.
Use the DBT until the amp settings are good.

A quick check of the outputs is done using the DMM set to ohms and looking for continuity between the case and two legs. A bad (Shorted) output will indicate a measurement similar too the reading when touching the two DMM leads together. (00.03) ohms or so.

 

[darkhosis]

I think I got it. I'll work on it today.

Can you help me understand what is going on when I pulled Q8 : the polarity flips, the measured output at the speakers goes down by half - but when I diode test the transistor it is behaving normally?

 

[Bassblaster]

Ugh, another SX-636 with high voltage at the outputs. jesus thats 3 i have seen with that issue within a month. mine, yours and another guys i can't think of his name right now.
remind me to never get another 70's pioneer receiver again.
EDIT: Well I feel like a total ass (not like i dont always feel like one anyway). just reread and saw this was a repost. well then.

 

[zebulon1]

The set is 45 years old.

 

[Bassblaster]

The set is 45 years old.

i am aware. i have a 636 as well. it doesnt work, but i own one.

 

[zebulon1]

The red (center) speaker terminal is the balance point in the circuit. One side of the amp circuit is positive and the other side is negative. The voltages are equal plus and minus. They meet on that center speaker terminal. If one side is weaker (less voltage) then the other polarity will win the balance battle.

 

[darkhosis]

Hey guys, I'm compiling my order from Mouser and using the parts list I found. I feel like I need to replace Q10 (2SA777) and Q8 (2SC1509) but I can't find equivalents.

Where do I go to find this info?

 

[Bassblaster]

Original---------------------Mouser Replacement

2SA777 --------------------512-KSA1220AYS
2SC1509 ------------------512-KSC2690AYS
2SC1451 ------------------512-KSC3503DS
2SD371--------------------863-MJ15015G
2SB531--------------------863-MJ15016G
the factory D371 and B531 are most likely fine, but those are the replacements for them.

2SA726---------------------MATCHED 512-KSA992FTA
The 2SA726 replacements need to be matched pairs as they are for the differential

 

[darkhosis]

Thanks. How did you find the info?

 

[Bassblaster]

from my thread lol

 

[darkhosis]

hah. By "matched" you mean I should replace both sides

 

[Bassblaster]

not exactly, left and right both have a pair of those. if you're going to replace the pair on your problem side (right?) you have to replace them both with matched new transistors. and matched means they both have a very close HFE rating.

 

[darkhosis]

sorry, I'm going on 65 hours work. that makes perfect sense

mouser doesn't offer matching, does it? do i test them myself?

 

[Bassblaster]

sorry, I'm going on 65 hours work. that makes perfect sense

mouser doesn't offer matching, does it? do i test them myself?

Yep. you match them yourself. Pete has more info on that.